A single semiconductor device of H-type bridge circuit 10 shown in FIG. 3 comprises first and third transistors 1 and 3 of high voltage side and second and fourth transistors 2 and 4 of low voltage side. First and second junctions A1 and A2 are provided respectively between an emitter electrode of first transistor 1 and a collector electrode of second transistor 2, and between an emitter electrode of third transistor 3 and a collector electrode of fourth transistor 4, and connected between first and second junctions A1 and A2 is an electric load 6 such as a cold cathode fluorescent lighting discharge tube driven by AC power.
In operation of bridge circuit 10, first and fourth transistors 1 and 4 and second and third transistors 2 and 3 are alternately turned on and off for alternate switching to cause adverse electric currents I1 and I2 to flow by turns through junctions A1 and A2 to activate load 6. Thus, switching operation of first to fourth transistors 1 to 4 converts DC power into AC power to turn on the discharge tube connected between junctions A1 and A2.
A prior art single semiconductor device comprised of H-shaped bridge circuit 10 shown in FIG. 3 is disadvantageous due to the increased plane size of the device because it requires a support plate (not shown) of larger size to mount all of first to fourth transistors 1 to 4 and a control IC therefor on a common plane of the support plate. To overcome such a disadvantage, for example, Japanese Patent Disclosure No. 55-111151 presents a semiconductor device which has two semiconductor elements in the vertically layered structure to reduce the occupation area. However, if power semiconductor elements are vertically layered in H-shaped bridge circuit, they inconveniently produces intensive heat during the operation without the desired heat radiation, causing deterioration in electric property of the semiconductor elements.
Accordingly, an object of the present invention is to provide a semiconductor device which has a plurality of semiconductor elements layered in a smaller occupation area and operable with the desired heat radiation.